[EE] 24-bit A/D. Are We in the Twilite Zone Here?
Tom Handley email (remove spam text)
Dan, I'm currently working on a PIC-based DAQ system that uses the
LTC2400. Others have mentioned noise problems and techniques with
low-voltage measurement. I think the key here is that the LTC2400 family are
extremely accurate converters that can provide very accurate measurements at
lower resolutions. I'm using 16-18 Bits for display while averaging all the
Bits. The software provides for calibration.
Wagner mentioned reference noise which is a good point that is sometimes
overlooked. I'm using a MAX6341 4.096V reference with a 1ppm/C Tempco, 0.02%
initial accuracy, and around 2.5uV of noise. It provides a pin for an
external cap which augments wide-band noise reduction. With 24 Bits the
resolution is ~244nV. As is pointed out in one of Analog Devices'
applications books; 600nV is the Johnson Noise in a 10KHz BW of a 2.2K
Resistor @ 25C. So, yes, this really is the "Twilight Zone" ;-)
The LTC2400 is extremely easy to talk to but you must decide which mode
to use. In my case, I use the External SCK/Single Cycle/Reduced Data mode.
With /CS initially High and SCK initially Low, the basic flow is:
Wait for End of Conversion:
- Set /CS Low
- Wait for SDI Low
Shift-in first 4 Bits (EOC -> Dummy -> Sign -> Extended Range)
At this point, you have Bits 31..28 in Data register Bits 3..0 where:
Bit 31 (3) = End of Conversion Bit
Bit 30 (2) = Dummy Bit
Bit 29 (1) = Sign Bit
Bit 28 (0) = Extended Range Bit
The next Bit is the MSB of the conversion where Bits 27..4 are the 24-Bit
result and Bits 3..0 are sub LSB's that can be ignored or used in averaging.
At 02:49 PM 4/24/00 -0600, Dan Michaels wrote:
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See also: www.piclist.com/techref/io/atod.htm?key=a%2Fd
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